ISA 18.2, Management of Alarm Systems for the Process Industries.
ISA 18.2 is the international standard governing the lifecycle management of alarm systems in DCS, SCADA, and PLC platforms across the process industries. It defines the alarm philosophy document, the rationalization process, determining whether an alarm is needed, what priority, what setpoint, what operator action, the design and implementation rules, basic alarms, advanced alarming, suppression rules, the operations expectations, target alarm rates, target priority distribution, and the maintenance and audit cadence. IEC 62682 is the international adopted version.
What ISA 18.2, Management of Alarm Systems for the Process Industries means.
Before ISA 18.2, 2009. Substantially revised 2016, alarm management was treated as a configuration task at DCS commissioning rather than a lifecycle engineering discipline. The standard codified what major operators had learned through alarm-flood incidents, Buncefield, Texas City, Longford. A control room cannot effectively respond to more than approximately 6-12 alarms per hour per operator. An alarm distribution skewed toward high and emergency priorities exceeds operator capacity. Alarm suppression and shelving rules must be designed into the alarm philosophy rather than applied as workarounds. ISA 18.2 sets a structured process for getting alarm management from current state to a sustainable target state and keeping it there.
What is the alarm rationalization process.
Rationalization is the per-alarm engineering review that determines whether the alarm exists, what priority it carries, what setpoint triggers it, what consequence accrues if the operator does not respond, and what action the operator is expected to take. Each rationalized alarm gets a documented basis, the documented reason the alarm exists that survives the next P&ID revision or DCS configuration migration. ISA 18.2 recommends rationalization by a multi-discipline team, process, operations, controls, process safety working through each alarm in the system in priority order.
What target distribution does ISA 18.2 recommend.
ISA 18.2 recommends an approximate distribution of 5% emergency, 15% high, 80% low and medium combined. The target reflects operator response capacity. Emergency alarms demand immediate dedicated action. High alarms demand action within minutes. Low and medium alarms are informational or contextual. A system skewed toward higher priorities, 15% emergency, 30% high typically reflects priority inflation, where each tag's owner argued for the highest priority their service could justify. Re-rationalization brings the distribution back to target.
How do suppression and shelving rules work.
Suppression rules suppress alarms automatically based on plant state, an alarm only meaningful in a particular operating mode is suppressed in other modes or based on cause-and-effect, a downstream alarm consequent to an upstream alarm is suppressed for the duration of the upstream alarm. Shelving is an operator action that temporarily suppresses a non-meaningful alarm with a documented reason and an expiry. State-based suppression rules are designed and tested at commissioning. Shelving rules are configured into the alarm system to enforce documentation and expiry.
Alarm performance targets published in ISA 18.2
Measured per operator console. These are the standard's own target and maximum manageable figures; a site sets its own in the alarm philosophy, and then measures against them.
| Metric | Target | Maximum manageable | What a failure looks like |
|---|---|---|---|
| Alarms per day | About 150 | About 300 | The operator stops reading and starts clearing |
| Alarms per hour | About 6 | About 12 | The alarm list is never empty |
| Alarms per ten minutes | About 1 | About 2 | Continuous annunciation |
| Peak alarms in any ten minute period | Ten or fewer | Ten or fewer | A flood the operator cannot triage |
| Percentage of ten minute periods carrying more than five alarms | Under 1 percent | Under 1 percent | Floods are routine rather than exceptional |
| Contribution of the ten most frequent alarms | Between 1 and 5 percent of the total | Under 5 percent | A handful of bad alarms is the whole system |
| Chattering and fleeting alarms | None, and each one carries an action plan | None | The operator learns to ignore a whole class |
| Stale alarms, standing more than 24 hours | Fewer than five on any day, each with an action plan | Fewer than five | The display is permanently red |
| Annunciated priority distribution, three priorities | About 80 percent low, 15 percent medium, 5 percent high | Same | Everything is high priority, so nothing is |
| Annunciated priority distribution, four priorities | About 80 percent low, 15 percent medium, 5 percent high, under 1 percent highest | Same | Same |
The ISA 18.2 alarm state model
Each state is entered and left by a defined event. Shelving, suppression by design and out of service are three different things, and the standard keeps them apart because they carry different authorisations.
| State | How it is entered | How it is left | Annunciated to the operator |
|---|---|---|---|
| Normal | The process is within limits and the alarm is enabled | The alarm condition becomes true | No |
| Unacknowledged alarm | The alarm condition becomes true | The operator acknowledges, or the condition clears | Yes, audible and visual |
| Acknowledged alarm | The operator acknowledges while the condition is still true | The condition clears | Yes, visual |
| Return to normal, unacknowledged | The condition clears before the operator acknowledged it | The operator acknowledges | Yes, visual |
| Shelved | The operator shelves a nuisance alarm, for a bounded time and on a shelving list | The time expires, or the operator unshelves it | No, but it appears on the shelved list |
| Suppressed by design | A designed state based or logic driven suppression becomes active | The suppressing condition ends | No |
| Out of service | Maintenance removes it under an authorisation | It is returned to service | No |
Common questions
What is the difference between ISA 18.2 and EEMUA 191.
What is alarm priority inflation.
Can the rationalization workbook drive the DCS bulk alarm load.
What does ISA 18.2 say about alarm flooding.
Get the I/O list template.
Fourteen columns with the signal class as a dropdown, auto-filter on every column, frozen header. Plain .xlsx.